Q87. Acoelomates are characterized by
- The absence of cavity surrounding the internal organs
- The presence of huge body cavity, as in case of terrestrial animals
- The presence of air sacs, as in case of birds
- The absence of brain in a group of extinct species
Acoelomates lack a fluid-filled body cavity called a coelom between the gut and body wall, filled instead by mesenchyme tissue. This solid body plan is seen in flatworms like Platyhelminthes. It impacts organ protection and movement compared to coelomates.
Correct Answer
Acoelomates are characterized by the absence of cavity surrounding the internal organs.
The coelom is a true body cavity fully lined by mesoderm; without it, organs embed directly in mesodermal tissue.
Examples include planarians and nemerteans with bilateral symmetry but no coelomic space.Option Analysis
Absence of Cavity (Correct)
No coelom means solid mesenchyme fills space between ectoderm, endoderm, and digestive tract.
This provides structural support but limits organ independence and shock absorption.
Triploblastic organization persists without cavity formation.Huge Body Cavity
This describes coelomates like annelids or arthropods with large fluid-filled cavities for organ suspension.
Terrestrial animals such as earthworms exemplify this, opposite to acoelomates.
Not a feature of solid-bodied acoelomates.Air Sacs in Birds
Air sacs form a respiratory system in birds for efficient gas exchange, unrelated to body cavity types.
They derive from lung outgrowths, not coelomic development.
Irrelevant to acoelomate classification.Absence of Brain
Some extinct species lack brains, but this does not define acoelomates.
Acoelomates like flatworms have simple brains (cerebral ganglia).
Neurological traits vary independently of coelom presence.Option Defines Acoelomates? Key Trait Example Group Absence of cavity Yes Solid mesenchyme body Platyhelminthes Huge body cavity No Coelomate feature Annelids, vertebrates Air sacs No Bird respiration Aves No brain No Neurological variation Not acoelomate-specific This body plan distinction is crucial for understanding animal phylogeny and evolution.
